(a) Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof and, more particularly, to a liquid crystal display performing frame rate control and a driving method thereof.
(b) Description of the Related Art
Flat panel displays such as liquid crystal displays (LCDs) have been developed and substituted for cathode ray tubes (CRTs) since they are suitable for recent personal computers and televisions, which become lighter and thinner.
An LCD representing the flat panel displays includes a liquid crystal panel assembly including two panels provided with two kinds of field generating electrodes such as pixel electrodes and a common electrode and a liquid crystal layer with dielectric anisotropy interposed therebetween. The variation of the voltage difference between the field generating electrodes, i.e., the variation in the strength of an electric field generated by the electrodes changes the transmittance of the light passing through the LCD, and thus desired images are obtained by controlling the voltage difference between the electrodes. A typical LCD includes thin film transistors (TFTs) as switching elements for controlling the voltages to be applied to the pixel electrodes, and a plurality of display signal lines for transmitting signals to be applied to the TFTs.
The LCD receives N-bit red (R), green (G) and blue (B) data from an external graphic source. A signal controller of the LCD converts the format of the RGB data, and a driving integrated circuit (IC) of the LCD selects analog gray voltages corresponding to the RGB data. The selected gray voltages are applied to a liquid crystal panel assembly, thereby displaying images.
The bit number of the RGB data input into the signal controller from the graphic source is usually equal to the bit number of data capable of being processed at the driving IC. Currently available LCD products usually process 8-bit data using driving ICs capable of processing 8-bit RGB data, which costs high. Therefore, in order to design a cost-effective LCD, it is required to select a driving IC having a capability of processing the data with the bit number smaller than eight.
In this connection, it has been proposed that frame rate control (FRC) should be applied for use in the LCD. The FRC reconstructs frame data such that an LCD having several driving ICs processing (N-M)-bit data displays images using only (N-M) bits among the N bits of an N-bit input RGB data, where M indicate the bit number of the lower bits of the input RGB data. The FRC converts the N-bit input data into an (N-M)-bit data such that among consecutive 2M frames, the number of frames where the converted data has a gray ‘A’ indicated by the upper (N-M) bits of the input data and the number of frames where the converted data has the next higher gray ‘A+1’ are regulated based on the lower M bits of the RGB data. Furthermore, the FRC converts the N-bit input data into a predetermined number of (N-M)-bit data respectively assigned to pixels in a group of the predetermined number of pixels such that the total number of pixels displaying the gray ‘A’ and the total number of pixels displaying the gray ‘A+1’ during a predetermined number of frames are regulated depending on the lower M bits of the RGB data. Since human eyes recognize spatio-temporal average of the gray of the (N-M)-bit data, the image appears the same as that represented by the N-bit data. Consequently, 2M additional grays between the grays of ‘A’ and ‘A+1’ can be displayed.
For example, let us consider an 8-bit input data with six upper bits and two lower bits. The 8-bit data can represent 28 (=256) grays ranging from ‘0’ to ‘255’. The upper 6 bits of the input data representing the highest four grays ‘255’, ‘254’, ‘253’ and ‘252’ are equal to ‘111111.’ Since there is no 6-bit number larger than ‘111111’ by one, the FRC cannot be applied to these data and thus the input data representing any one of the highest four grays should be represented by a single 6-bit data ‘111111’ for all the frames. This causes gamma degeneracy for the highest four grays. Then, each of red, green and blue colors has only 253 grays, the total number of colors obtained by mixing these primary RGB colors is 253×253×253 (=16,194,277), which is smaller than the number of colors obtained by mixing the primary colors having entire 256 grays, i.e., 256×256×256 (=16,777,216), by about six hundred thousand.
Meanwhile, a conventional LCD with FRC has deteriorated image quality. For instance, when a lower part of a display screen displays a black image while an upper part of the screen displays an image with increasing or decreasing grays along a vertical line to have maximum brightness for each of red, green, blue and white colors, a plurality of horizontal lines are displayed every four grays, and this seriously deteriorates the picture image quality. Such a phenomenon seems to be generated due to frame inversion together with the FRC.